A number of operations within a borehole can be carried out with coiled tubing which is temporarily inserted into the borehole from the surface. The tubing can serve as a pipeline for fluid which is being delivered into the borehole and/or can carry equipment attached to the downhole end of the tubing. Some operations make observations or measurements. For example an appropriate sensor fitted to the bottom end of coiled tubing can measure natural radioactivity from the surrounding rock formation. Communication from the sensor at the bottom end of coiled tubing to the surface can be provided by an electrical cable or can be provided by an optical fibre, with signals from the sensor converted into light signals directed along the optical fibre. By way of example Society of Petroleum Engineers paper SPE134389 describes the use of optical fibre to provide communication from a downhole position sensor to the surface.
For some operations, a hydraulic motor and a cutting tool such as a drill bit, a mill or a reamer, are fitted to the downhole end of the coiled tubing so that fluid pumped down the tubing powers the hydraulic motor which in turn rotates the cutting tool. A hydraulic motor and a rotary cutter which is a mill may for instance be used to mill through a packer or plug temporarily placed in a borehole to close off one part of the borehole from another. For example US2009/0071645 mentions a wellbore tool which may comprise a milling bit driven by a hydraulic motor at the bottom end of tubing. A load detection sub assembly at the bottom end of the tubing can be used to detect local changes indicative of bit stalling. Data from this sub assembly and from other sensors downhole is communicated to the surface by means of a communication line which is optical fibre.